1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to the field of variable speed wind turbines, and more particularly, to a variable speed wind turbine comprising a doubly fed induction generator (DFIG), an exciter machine, an intermediate static converter not connected to the grid and a control system to keep the doubly fed induction generator connected to the grid during a low voltage event, and a method implementing the same.
2. Description of the Related Art
In the last few years, wind power generation has increased considerably worldwide. For this reason, grid regulation companies have modified wind turbine electrical grid connection specifications in order to avoid disconnecting a wind turbine from the grid when a low voltage event or some kind of disturbance occurs in the grid. Thus, other new requirements are demanded of the wind turbines with respect to their contribution to the grid's stability when voltage disturbances occur.
Normally, when a grid fault occurs in a doubly fed system, the over-current converter protection switches-off the converter. This protection is activated because the rotor current cannot be regulated by the rotor side converter due to the short circuit which occurs in the stator side of the doubly fed generator. However, this switching disabling is not enough to protect the system because the rotor current flows thorough the converter diodes to the DC Bus circuit, increasing the DC BUS Voltage. This over voltage could damage the converter components. For this reason, the rotor is short circuited and the stator of the generator is disconnected from the grid. This type of control has been implemented in doubly fed wind turbine systems until recently. However, the growth of wind power generation is forcing the creation of new grid code specifications, so the wind power generation must adapt to these new requirements. These requirements are focused on two main points: no disconnection of the wind turbine from the grid and the wind turbine's contribution to the grid stability.
Many solutions have been developed by the different wind turbine manufacturers in order to satisfy the new grid code requirements. Some of these solutions are described in the following documents:                U.S. Pat. No. 6,921,985: This document shows a block diagram where the inverter is coupled to the grid. An external element from the converter like a crowbar circuit is coupled with the output of the rotor of the generator. This crowbar circuit operates to shunt the current from the rotor of the generator in order to protect the power converter when a grid fault happens and to keep the system connected to the grid.        US 2006/016388 A1 This document shows a block diagram where the inverter is coupled to the grid. An external element from the converter like a crowbar circuit is connected to the rotor of the generator. This crowbar circuit is used to electrically decouple the converter from the rotor windings when a low voltage event occurs.        U.S. Pat. No. 7,102,247: This document shows two block diagrams with different configurations. Both of them show a converter connected to the grid (V1, V2, and V3). Two external elements are connected in order to maintain the system connection to the grid when a grid fault occurs. In this document, a crowbar circuit with resistance is shown and some extra elements are included in the BUS system. These additional elements are activated when a grid fault occurs.        WO 2004/098261: This document shows a block diagram where a converter is connected to the grid. This document shows the crowbar circuit connected to the BUS system. This crowbar circuit is activated when the BUS voltage rises after a low voltage event.        
However, every solution developed and described in these documents and in other documents such as WO2004/040748A1 or WO2004/070936A1 has a common feature: all the solutions include power electronic converters directly connected to the grid. This feature is the source of a very important issue when a transient voltage occurs in the grid. As will be explained, this grid side converter presents a functional limitation when a fault occurs, because the grid side converter is going to operate with a reduced grid voltage (depending on the grid fault), so its energy evacuation capacity is reduced. Currently, when a grid fault occurs, the generator demagnetizing energy is sent to the BUS and due to the grid side converter limitation, the BUS voltage rises and could damage converter components. For this reason, these solutions include some extra elements connected mainly to the rotor or BUS system. These extra elements absorb the generator demagnetizing energy when a grid fault occurs in order to keep the wind turbine connected to the grid and, thus, satisfy the new grid code specifications. All these elements are normally formed from a combination of passive elements, like resistors, and active elements, like switches.
In these types of solutions, every disturbance or fluctuation occurring in the grid directly affects the grid side converter, so its current limitation implies that the performance of the wind turbine during a grid fault is not completely optimized.